This invention is in the field of liquid sampling techniques, as used for example in liquid chromotography equipment or in blood testing equipment. The invention is an improvement in the method of introducing liquid samples into the equipment where a valve having at least six ports is used to perform sample loop injections.
The typical liquid sampling apparatus is as shown in FIG. 1.
The prior art method for doing sample loop injections with a typical liquid sampling apparatus is as follows as shown in FIG. 1A and FIG. 1B.
1. The plunger 29 of syringe 4 pushes forward a bit, with the six port valve 6, at "FILL".
2. The plunger 29 pulls a small air bubble 8 into the needle 10 in the sample tower 12.
3. The vial 14 is lifted into the sample tower 12.
4. The plunger 29 pulls the exact amount desired from the vial 14 (plus a small quantity such as 0.6 .mu.L to account for loss on the walls of the tubing 16).
5. The vial 14 is lowered.
6. The plunger 29 then pulls the sample 18, with air bubble 8 ahead of sample 18, the precise distance into the sample loop 20.
7. The six port valve 5 is thrown to "INJECT" so the sample 18 can be analyzed in the remaining part of the equipment, including analytical column 21, as shown in FIG. 1B.
There are several possible problems associated with this routine. The first is the introduction of an air bubble into the sample loop 20 behind the sample slug 18. There is also the possibility of introducing unwanted air into the sample loop 20 ahead of the sample 18 if the sample 18 is pulled more than the exact amount. Air bubbles can be broken up and dispersed at connector junctions 22 causing inaccuracies in the sample draw.
The second is the possibility of leaving part of the sample 18 on the walls of the transport tubing 16 since this tubing 16 is not "swept" with solvent (hence the 0.6 .mu.L offset) Also, to draw the correct amount of sample 18, the flush solvent 26 used to clean the system between samples must be completely degassed. Otherwise, there is the possibility of cavitation or outgassing when the plunger 29 is drawn back. This creates air bubbles which can make the volume of sample 18 drawn very inaccurate.
A major source of error concerns the design of the mechanical syringe drive 28. Even on a brand new system, where there is very little play in the screwdrive (not shown), the shuttle (not shown) that holds the plunger 29 can wobble when the lead screw (not shown) rotates. Over a period of time, the drive 28 will wear and more "slop" is introduced. This affects the accuracy of the volume of sample 18 drawn as well as the reproducibility of the sample volume over time.